Effective heat insulating pipe coverings employing materials of relatively low density and having a capacity for resisting relatively high temperatures (for example, on the order of 650.degree. F.) have grown in importance in recent years. Such pipe coverings are exemplified by the patents to Stephens et al, U.S. Pat. Nos. 2,778,759; 2,790,464; 2,946,371 dated Jan. 22, 1957, Apr. 30, 1957 and July 26, 1960 respectively.
Siliceous insulating compositions in general and particularly glass fibers, have long been noted for their insulating value. However, depending upon the end use of the insulation, there are disadvantages to be found in glass fiber insulation now in general use. It is characteristic of insulating mats typified by the Stephens et al patents, supra, that the respective fibers in the mat are bonded to one another by a suitable resin such as a phenolic resin. The use of such a binder, or of other resin binders, places a limit on the temperature the insulation can withstand without structural and chemical degradation. For example, a pipe carrying a fluid whose nominal temperature is 650.degree. F. and having a resin bonded glass fiber mat in contact therewith normally experiences a burning of the binder on the hot face of the insulation, i.e., the surface contiguous with the pipe. Sustaining punking of the binder may occur depending upon the quantity of resin employed for binding and the critical heating temperature of the resin used. Punking, of course, is a term of art used to denote the comparatively rapid flameless oxidation of the binder with generation of heat. Odors and fumes given off by such thermal decomposition are offensive and are capable of discoloring and staining adjacent materials. Furthermore, punking may be associated with exothermic reactions which increase temperatures through the thickness of the pipe covering causing a fusing or devitrification of the glass fibers and possibly creating a fire hazard. Once devitrification has occurred the insulation is usually incapable of thermally insulating its associated pipe and may warp and pull away from the very pipe it was intended to insulate. Finally, devitrification of the glass in the pipe insulation causes the product to lose its structural integrity to the extent that vibrations and impacts occurring during normal usage may cause dusting problems. In an extreme case the normal vibrations and impacts may dislodge the pipe insulation causing it to become a personal safety hazard in the working environment.
Various attempts have been made to provide high temperature pipe insulations. One such attempt is illustrated in U.S. Pat. No. 3,053,715 issued Sept. 11, 1962 to Labino which discloses an insulating unit having a combination of bonded and unbonded layers of siliceous fibers. The fibers in the inner core or the fibers adjacent the surface to be insulated are sub-micron in size so that they are self-adhering and do not require a binder. Preferably the un-bonded sub-micron fibers are leached so that they consist essentially of silica. Labino stated that a high temperature binder could be used to bond the sub-micron fibers in certain circumstances provided, of course, that the minimum punking or critical temperature of the binder is above the maximum temperature of the heat source being insulated. If any of the layers did have a binder it was found that a binder content between 10-25% based on the ignition loss was satisfactory.
While the high temperature pipe insulation disclosed by Labino can be very useful, the product is primarily disadvantageously characterized by the high cost which is incurred by the manufacturing steps of making sub-micron and/or leached glass fibers. Such high cost precludes use of the products of Labino in the area of 850.degree. F. pipe insulation.
Another illustration of an attempt to provide a high temperature insulation product useful for pipe insulation is described in U.S. Pat. No. 3,846,225 issued Nov. 5, 1974 to Stalego. The glass fiber insulating materials disclosed by Stalego utilizes a nonpunking organic binder material to bond glass fibers together. The resultant bonded mass is saturated with an inorganic binder-saturant which hardens under treatment to provide a ceramic-like heat resistant coating for the glass fibers in addition to auxiliary bonding. Unfortunately, this product may be characterized as dusty and as having low mechanical or structural integrity. Possibly, the boric acid present in the inorganic-binder saturant may corrode the pipe about which the insulation may be disposed.
Other methods which heretofore have been attempted in order to increase the punk resistance of the binder systems and to more nearly align the properties of the binder system used with the properties of the glass fiber include the reaction of nitrogenous substances such as melamine, dicyandiamide, urea, thiourea, biurea, guanidine and similar compounds with phenol-aldehyde partial condensation products of the resole type. Although the incorporation of such nitrogenous compounds improves the punk resistance and overall thermal stability of the binder system, products composed of glass fibers in association with such binder systems are still not suitable for use in environments approaching the limits of the heat stability of the glass fiber itself. Other methods intended to overcome the comparatively low thermal stability of phenolic resole binder systems include the mixture thereof with other resinous systems, such as alkyds, or the use of water glass or sodium silicate as the binder system. The latter method is, however, disadvantageous in that the high sodium ion concentration in commercially available sodium silicates leads to attack of the glass fiber by the sodium ion and subsequent deterioration of the fibers, with the result that the products formed are weak and subject to many of the limitations present in the use of cementitious materials from the standpoint of strength, flexibility and brittleness.
Higginbottom in U.S. Pat. No. 3,956,204 dated May 11, 1976 demonstrated an antipunking phenolic resin binder system useful for fibrous thermal insulations of low density. However, this conventional antipunk system when used in 850.degree. to 900.degree. F. pipe insulation use environment still experiences punking or burn out and exotherms to greater than 1200.degree. F. This high temperature causes the bonded glass to soften and devitrify resulting in a separation of the glass fibers at their points of contact. In a worse case this high temperature can cause a distinct fire hazard as described above.